An introduction to astronomy: designed as a text-book for the use of students in college. By Denison Olmsted ...

226'114 PLANETS.
called the jA'Iements qf itS Orbit. They are seven in 1 tunlber:
of whlich the first two determine tile position of tile plalle of
tihe orbift, and the otlher five relate to thle orbit and thle p)lalet
in that pIlalne. TiGSC elements are:
1...l position of ti/th line qf the tnodes.
2. 77e zInclinatiow to thoe ecliptic.
3. 7fte, l)riodiO tine.
4. /T"e man. distancef jo'i the sun, or sermi-aceis nmaj-or.
5. The ec1eentietty.
6.'The 1)tlace (f tole pI),erihelionm.
I. flie place qfI/ toplantet bt its bit; a2t a lpaticular epoc.
368. It may at first view be supposed that vwe can proceed
to find the elements of the orbit of a planet inl the samnie manner as we did tllose of the solai' or llunar orbit, namnlly, by observations on the right ascension and deelination of the body,
cotvertced into latitttudes and longitudet s by means of spherical
t:rigonometry (sce Art. 132). But in tlhe case of thle moon,
we are sittluated ill thle center of her imotions, and tle atpparent
coincide vithl theo real motions' and il resl)ect to the stilln, ou1r
lbservattio ls on hiis atar p-rInt motions. give us tihe earth's r'eal
motionls, allowing 180 differellnce ill longitude. IBut as we
have ahlread)y seen, tile motions of the planets app)ear exceedingly different to ius, frolm what tley would if seen from tlhe
center of thleir motions. it is ne1essay'Y, therefore, to deduce'rom observations mlltta on the earth the corresl)onditlg results
as they would be if viewed fi'oml the center of the sun; tllhat is,
il the blan nlguae of astrllolnoe1rs, hlavin tle yeocentric place of,t planlet, it is required to find its ht'elioenbtic place.
"369. 1The first steps inl this pr1ocess are the same as in the
ease of tlhe sunt and mtoon. That is, for the purpose Cf findiltng
tie riglht ascension and declination, the pllatet is observed on
thle lc ridian l with the1'Tlansit Instrument and M:ural Circle
(see Arts. 155 andl 230), and firom these otbservations, the
}plane1t':s qeocentrie longitde and lI atitude are compl)ted by
spl icrical trigonllmetiry. The distancee of the pl)lanet firot tihe
sun1 is known nearly by' cpler's lawv. lFrm these (ata it is
required to timid the heliocentric longitiude and latitutlde.
L]et S al(l ], (Fig.'71) be the sun atlnd earth, A80:II[ thle